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GRB 090423
Afterglow of GRB 090423 taken by the Gemini Observatory
Detection
Detection time	07:55 UTC April 23, 2009
Detected by	SWIFT
Duration	10 seconds
Position
Right ascension	09h 55m 33.08s
Declination	+18° 08′ 58.9″
Redshift	8.0 ≤ z ≤ 8.3
Distance	13 × 109 lightyears
Energetics
See also: Gamma-ray burst, Category:Gamma-ray bursts

GRB 090423 is a gamma-ray burst (GRB) detected by the Swift Gamma-Ray Burst Mission on April 23, 2009 at 07:55:19 UTC. The afterglow of GRB 090423 was detected in the infrared, and allowed astronomers to determine that the redshift of GRB 090423 is z = 8.2, which makes GRB 090423 the current record holder for most distant object detected.

A gamma-ray burst is an extremely luminous event flash of gamma rays that occurs as the result of an explosion, and is thought to be associated with the formation of a black hole. The burst itself typically only lasts for a few seconds, but gamma-ray bursts frequently produce an "afterglow" at longer wavelengths that can be observed for many hours or even days after the burst. Measurements at these wavelengths, which include X-ray, ultraviolet, optical, infrared, and radio, allow follow up study of the event.

That the speed of light is finite also means that GRB 090423 is also the earliest object ever detected. The universe was only 630 million years old when the light from GRB 090423 was emitted, and its detection confirms that massive stars were born and dying even very early on in the life of the universe. As the earliest object detected, GRB 090423 provides a unique tool for studying the early universe, as few other objects are bright enough to be seen with today's telescopes.

Discovery and observation

On April 23, 2009 at 07:55:19 UTC the SWIFT satellite detected a burst that lasted 10 seconds and was located in the direction of the constellation Leo.^[1][2]

GRB 090423: The oldest and the farthest known object in the Universe

Swift localized the field in which GRB 090423 occurred, and 77 seconds after the burst, the Swift UVOT Photometric System took a 150 second exposure of the field, but was unable to detect an optical or ultraviolet afterglow.^[2] A few minutes after its discovery, ground based telescopes began observing the field. Within 17.5 hours of the burst, Nial Tanvir’s team found an infrared source at the Swift position using the United Kingdom Infrared ISAAC Telescope on Mauna Kea, Hawaii.^[3] They observed a drop off in flux beyond 1.13 micron. Attributing this drop off to Lyman alpha absorption by neutral hydrogen in the intergalactic medium, they calculated a redshift of 8.2 for GRB 090423.^[4] The team of C.C. Theone and Paolo D'Avanzo observed the afterglow of GRB 090423 using the Italian TNG 3.6m telescope located in the Canary Islands, Spain.^[5] They obtained two hours of spectra, which when combined, suggested a very weak signal at the position of the afterglow. They too saw a drop off in flux near 1.1 micron, and reported a redshift of 8.1 for GRB 090423, which is consistent, within error, of the redshift reported by Tanvir et al.^[6]

The intergovernmental astronomy organisation, European Southern Observatory (ESO) also detected the faint GRB using ESO’s Very Large Telescope.^[7] They confirmed the same redshift data collected and approximated the event to have taken place 13 billion years ago. The GRB was not visible in Chile when SWIFT first detected the burst at 07:55 UTC, but was the following day at 03:00 UTC, which allowed the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) at La Silla Observatory to make observations of the burst, and find a redshift consistent with the value reported elsewhere.^[8][9] The last observers to gather data during the event was the Combined Array for Research in Millimeter-wave Astronomy (CARMA) observatory. The observation of GRB 090423 by CARMA was taken at a frequency of 92.5 GHz. While the afterglow was not detected, they were able to place a 3-sigma upper limit of 0.7 mJy on the flux density of the afterglow.^[10]

Observation history

Significance

At a redshift of z = 8.2, the burst is the current record holder for the most distant known object of any kind.^{[5][11][12][13][14][15][16][17][18]} GRB 090423 is also the oldest known object in the Universe, as the light from the burst took approximately 13 billion years to reach Earth.^{[19][20][21][22][23]} The burst occurred when the universe was approximately one twentieth of its present age. Prior to the observations done on GRB 090423, the previous record holder for age and distance was GRB 080913, which was observed in September 2008.^[24][25][26] That burst had a redshift of 6.7, placing it approximately 190 million light-years closer to Earth than GRB 090423. Derek Fox, who led the observations done by Pennsylvania State University, suggests that the GRB was most likely the result of the explosion of a massive star and its demise, which would probably have signalled the birth of a black hole.^[27] The event occurred roughly 630 million years after the Big Bang, confirming that massive stellar births (and deaths) did indeed occur in the very early Universe.^[28]

Joshua Bloom of the University of California, Berkeley, who was able to observe the afterglow at the Gemini South telescope in Chile, called the discovery of GRB 090423 a "watershed event" as it marked "the beginning of the study of the universe as it was before most of the structure that we know about today came into being." Nial Tanvir, who was part of the VLT team, suggests that gamma-ray bursts provide a unique tool to study the universe at early times because everything else is too faint to be observed. For instance, the first generation of stars have yet to be directly observed, but the progenitor of GRB 090423 may belong to this class. These early stars are expected to contribute to the reionisation of the universe, a process which ended at a redshift of about 6. As more powerful telescopes begin operation, such as the James Webb Space Telescope that is set to launch in 2014, astronomers hope to pinpoint the locations of faint GRB host galaxies by using blasts similar to that of GRB 090423.^[16]

References

External links

• Video of GRB 090423 on Youtube
• Slashdot article "Most Distant Object Yet Detected, Bagged By Galileo Scope"
• Swift Mission at NASA/Goddard Space Flight Center
• GRB 090423 on WikiSky

Coordinates: 09^h 55^m 33.08^s, +18° 08′ 58.9″